Semiconductor laser diodes (LDs) are well known sources for visible light in the red, green and blue (RGB), and are often combined in an RGB-module to make a color source.
In recent decades, the main thrust of the development of LDs and related light emitting diodes (LEDs) has been to refine the design of blue and green LDs (and white light emitters) around the gallium nitride materials system, GaAlInN, so that green and blue emitters are available to complement the red emitters that were developed earlier around the GaAlInAsP materials system. Important areas of the development have been to increase power, efficiency, lifetime and reliability. Efficiency is commonly stated as wall plug efficiency (WPE) which is the ratio of output optical energy to input electrical energy.
More recently, interest in direct retinal projection has increased for virtual reality and augmented reality applications. It is envisaged that an RGB projection source is incorporated into glasses or a visor as wearable technology. In contrast to a classic projection system, or other applications such as lighting and welding, where high power is needed, for direct retinal projection low power is needed. However, conventional LDs have not traditionally been optimised with low power applications in mind, and tend to be relatively unstable when operated at low powers, since low power operation means operation close to threshold.
FIG. 1A is a graph of light output power L plotted against drive current I (so-called L-I characteristic) of a typical commercially available edge-emitting blue LD. As can be seen, the threshold drive current is about 25 mA and increasing the drive current from threshold up to about 100 mA produces output powers in the 0 to 100 mW range.
FIG. 1B shows the wall plug efficiency (WPE) in percent as a function of drive current for the same LD as FIG. 1A. The LD is quite inefficient close to threshold at about 25 mA and first becomes relatively efficient at higher drive currents, with a WPE of about 20% being attained at a drive current of 40 mA. WPE then increases more slowly and saturates at about 30%.
It can thus be appreciated that operating a conventional edge-emitting LD at low power close to threshold in the sub mW output range will generally result in a very inefficient and unstable operation. Power stability with temperature is a particular problem, with a power variation of 0.3 mW per degree Centigrade being typical.
US 2015/0103404 A1 relates to a design of virtual reality or augmented reality projection glasses. The glasses incorporate an RGB-module comprising red, green and blue LDs. For the blue LD, an edge-emitter is disclosed which is based on a ridge design with the ridge being parallel with the c-plane of a GaN crystal. Different examples have cavity front mirror reflectivities Rf of: close to zero (no coating on output coupler facet); 50% and 75%. A range of cavity lengths of about 100 to 500 μm is considered. The ridge widths W are in the range 1-2 μm. The Rf=75% examples have cavity lengths L=30-80 μm in order to keep the WPE reasonably high. The Rf=50% examples permit the threshold current to be reduced to around 5 to 20 mA depending on the gain, albeit with reduced WPE. It is said that ‘slope efficiency’ ηd must remain high to achieve favourable WPE, where ‘slope efficiency’ is the ratio of optical output power to input drive current.